Staked fasteners, such as studs and rivets, are a well-known fastening technology. Studs are typically used to secure a body, such as a circuit board, to a foundation. Rivets, on the other hand, are typically used to fasten two bodies, such as sheet materials, together at an overlapping seam. For either a stud or a rivet, fastening is accomplished by the creation of a head on a shank. In the case of a stud having two ends, a head is formed on one end of the shank with the other end fixed to the foundation with the body therebetween. In the case of a rivet, heads are formed on both ends of the shank with the bodies therebetween.
A staked fastener can be made from many materials, such as steel, aluminum and plastic. Where weight is a concern, such as in an airplane, helicopter, or spacecraft, materials with high strength-to-weight ratios, such as plastic, are preferred. Unfortunately, plastic rivets simply do not have the ultimate strength of rivets made from other materials. As a result, plastic rivets, though desirable for many applications, cannot be practicably used, or in some cases used at all.
There have been many advances in the plastic rivet. From an initial beginning when plastic rivets were simply all plastic, such as a thermoplastic (e.g., polyetheretherketone (more commonly known as “PEEK”)), newer composite plastic rivets have some type of reinforcement added to the plastic. For example, many composite plastic rivets are reinforced with fibers, such as carbon or glass.
Rivets are typically formed in a two step process. First, an initial head is formed with a shank extending therefrom. Generally, the initial head is formed in a factory setting by a machine, by molding or staking an end of a rod, which tends to make a high quality initial head with a properly contoured contact surface (the surface of the head that will abut the body). Also, additional fabrication of the initial head, such as cleanup of the contact surface, can be performed relatively easily.
Then in an application, a second head is formed in situate on the end of the shank. More specifically, when a rivet is used in an application, such as connecting two bodies, a hole is made in each of the two bodies. The diameter of the holes is sized such that when the holes are aligned with each other the shank can pass through, but the initial head can not pass through the first hole that the shank passed through. The second head is then formed on the other end of the shank, thereby securing the two bodies together.
Unlike the initial head, the second head is formed at the job site, typically using a hand held tool, thus the conditions are not as controlled as those during the formation of the initial head. Additionally, cleanup of its contact surface is impossible, as it is abutting the body. As a result, the precision of the contact surface of the second head is generally not to the level of that of the contact surface of the initial head. This discrepancy results in a second head with less strength than the initial head.
More specifically, many processes used on plastic rivets to form the second head create voids in the contact surface of the rivet head. As a result, the head has a local weakness at its parameter edge. If pressure is applied thereto, it will cause a local failure of the head. These local failures result in “play” between the rivet and body, thereby creating the unacceptable amount of movement between the head and the shaft, causing the rivet to fail.
What is needed in the art is a staked fastener head and way to make the head such that the resulting rivet, or stud, is stronger than it otherwise would be.